This invention relates to the art of heat exchangers and, more particularly, to heat exchangers of the type having a fuel burner firing into a combustion chamber surrounded by an annular heat exchange passage through which fluid is circulated for receiving heat from the combustion chamber.
The invention is particularly applicable for use with a diesel fired coolant heating system and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and may be used for other purposes.
It is well known that diesel engines are difficult to start at relatively low temperatures. For this reason, it is common to leave a diesel engine idling for long periods of time so the engine will not cool down to the point where starting would be difficult. Such continuous idling wastes diesel fuel and a number of arrangements have heretofore been proposed for keeping a diesel engine warm without the necessity for continuously running the same. One prior arrangement includes a separate heat exchanger through which the diesel engine coolant liquid is circulated by means of a pump as shown, for example, in the commonly assigned U. S. Pat. No. 4,099,488 to Damon, the teachings of which are incorporated hereinto by reference. In the device disclosed by this patent, a diesel fuel burner fires into the combustion chamber of a heat exchanger. The water and antifreeze liquid coolant from the diesel engine is circulated by a pump through a heat exchange passage surrounding the combustion chamber with necessary thermostats and controls being provided to maintain the desired temperature of the diesel engine coolant liquid. The use of a diesel fuel burner in the heat exchanger thus makes it possible to employ the same fuel source as the diesel engine.
The heat exchanger there used includes elongated concentric inner, central and outer tubular members which define first and second elongated annular exhaust passages therebetween. The diesel fuel burner is mounted adjacent one end of the tubular members and discharges hot gases into the interior of the inner tubular member. These hot gases flow in one direction through the inner tubular member and then generally reverse direction by flowing through a plurality of openings spaced toward the opposite end of the inner tubular member and into the first annular passage. The hot gases then pass into the second annular passage adjacent the one end, generally reverse direction and flow back along this second annular passage. The hot gases are ultimately discharged through an exhaust outlet associated with the outer tubular member at the end thereof spaced remote from the burner. A housing surrounding the outer tubular member defines an annular heat exchange passage through which the diesel engine coolant flows. An arrangement of this type induces more complete combustion of the fuel-air mixture by providing a relatively long flow path for the products of combustion. However, and while the construction disclosed by the Damon patent has provided generally satisfactory results, it has been found that the inner tubular member deteriorates prematurely because of the manner and nature of the flow of hot gases in communication therewith.
Another heat exchanger construction is disclosed in U.S. Pat. No. 3,358,651 to Maruyama. In this arrangement, inner and outer tubular members are provided for defining an annular exhaust passage therebetween. Hot gases from a burner flow in one direction through the inner tubular member and then generally reverse direction to flow back through the annular passage. The inner tubular member is of double-wall construction and liquid is circulated through the space between these walls. As a result, the inner surface of the inner tubular member will always be at a relatively low temperature. This is unsatisfactory in a heat exchanger having a diesel fuel burner because the inner surface will soon be coated with carbon which greatly reduces the heat transfer characteristics and also reduces the combustion efficiency. The inner surface of the inner tubular member must be maintained at a highly elevated temperature in order to prevent formation of carbon deposits and for enhancing combustion of the fuel-air mixture at it travels through the inner tubular member. Also, the inner tubular member is of a generally conical configuration which makes it more difficult to fabricate.
In this Maruyama patent, the inner tubular member is laterally offset relative to the outer tubular member so the annular passage is not symmetrical about the inner tubular member longitudinal axis. However, the annular passage progressively decreases in size toward the exhaust outlet in a uniform manner and the portion of the annular passage disposed on the opposite side of the inner tubular member from the exhaust outlet always remains larger in area than the portion of the annular passage disposed on the side of the inner tubular member which faces the exhaust outlet. With an arrangement of this type, the exhaust gases follow the path of least resistance and simply flow longitudinally through the annular exhaust passage on the side thereof opposite from the exhaust outlet and do not cross over to the side of the exhaust outlet until the gases have traveled substantially the full length of the passage. Thus, the walls of the annular exhaust passage are not uniformly heated to a high temperature and inefficient heat transfer occurs from the wall of the outer tubular member to the coolant fluid flowing through the heat exchange passage.
The present invention contemplates new and improved apparatus which overcomes the foregoing specific problems and others and provides a new heat exchanger construction which is simple, effective, economical and particularly adapted to use in the environment of a diesel fueled engine coolant heater.